The invention relates generally to semiconductor optical waveguides and more particularly to a thick semiconductor optical waveguide that is substantially polarization independent, has increased coupling efficiency to optical fiber and superior uniformity and reproducibility.
Fiber optic communication systems have gained widespread acceptance over the past few decades. With the advent of optical fiber, communication signals are transmitted as light propagating along a fiber supporting total internal reflection of the light propagating therein. Many communication systems rely on optical communications because they are less susceptible to noise induced by external sources and are capable of supporting very high speed carrier signals and increased bandwidth. Unfortunately, optical fiber components are bulky and often require hand assembly resulting in Sawer yield and higher costs. One modern approach to automating manufacture in the field of communications is integration. Integrated electronic circuits (ICs) are well known and their widespread use in every field is a clear indication of their cost effectiveness and robustness. A similar approach to optical communication components could prove very helpful.
In an attempt to integrate optical components, manufacturers try to miniaturise optical systems within a single chip. For example, an InP structure can be formed on a substrate and can act as a waveguide for conducting an optical signal. Typically, the waveguide structure is thin and acts as a two dimensional waveguide, thereby effecting polarization of a signal guided therein. In order to provide polarization independence, several approaches exist.
Integrated wavelength multi/demultiplexers are important components for wavelength division multiplexing (WDM) optical communication systems. Integration offers the advantages of compactness, reliability, and reduced packaging costs. Further, implementation in a semiconductor material, particularly the InGaAsP/InP system important for optical fiber communications systems, would permit monolithic integration of these passive devices with active ones, such as lasers, modulators, optical switches, and detectors, resulting in sophisticated wavelength sensitive photonic integrated circuits with complex functionalities.
As described above, one of the major drawbacks in an integrated wavelength multi/demultiplexers is the polarization sensitivity of the device. Since an optical signal propagating through an optical fiber has an indeterminate polarization state, the switching/routing devices must be substantially polarization insensitive. However, planar waveguides usually have different propagation constants for TE (transverse electric) and TM (transverse magnetic) waveguide modes. For wavelength multi/demultiplexers, this difference in propagation constants results in a wavelength shift in the spectral response peak or the passband of each wavelength channel. This wavelength shift is sensitive to the design of the planar waveguide, and can be as large as 3 nm or more. As WDM systems are being designed towards smaller and smaller channel spacingxe2x80x94currently from 1.6 nm to 0.8 nm and even less in the future, even a small polarization dependent wavelength shift (e.g. 0.3-0.4 nm) is of concern.
Two types of integrated wavelength multi/demultiplexers that have been widely investigated art; phased waveguide arrays and etched grating-on-a-chip spectrometers. Grating based devices require high quality, deeply etched grating facets. The optical loss of the device depends critically on the verticality and smoothness of the grating facets. However, the size of the grating device is much smaller than the phased array and the spectral finesse is much higher due to the fact that the number of teeth in the grating is much larger than the number of waveguides in the phased array. This allows the grating based device to have a larger number of channels available over its free spectral range (FSR) and consequently can he scaled-up easily to high density operation.
In waveguide array based devices, several approaches have been used to compensate for the polarization sensitivity; for example the insertion of a half wave plate in the middle of the waveguides array is described by H. Takahashi, Y. Hibino, and I. Nishi, in a paper entitled xe2x80x9cPolarization-insensitive arrayed waveguide grating wavelength multiplexer on siliconxe2x80x9d, Opt. Lett., vol. 17, no. 7, pp. 499-501, 1992. Alternatively, the use or non-birefringent waveguides with a square cross section has been described by J. B. D. Soole, M. R. Amersfoort, H. P. Leblanc, N. C. Andreadakis, A. Raijhel, C. Caneau, M. A. Koza, R. Bhat, C. Youtsey, and l. Adesida, in a paper entitled xe2x80x9cPolarization-independent lnP arrayed waveguide filter using square cross-section waveguidesxe2x80x9d, Electron. Lett., vol. 32, pp. 323-324, 1996. Birefringence compensation using two different rib waveguides has been described by P. C. Chou, C. H. Joynerm M. Zimgibl, in U.S. Pat. No. 5,623,571 entitled xe2x80x9cPolarization compensated waveguide orating routerxe2x80x9d. In the ""571 patent the polarization compensation is not within the slab waveguiding region. This technique requires either two regrowth steps as described in the patent and in a paper by the same authors entitled xe2x80x9cPolarization compensated waveguide grating router on InPxe2x80x9d, Electron. Lett., vol. 31, pp. 1662-1664, 1995, or two etching steps as described by C. G, M. Vreeburg, C. G. P. Herben, X. J. M. Leijtens, M. K. Smit, F. H. Groen, J. J. G. M. van der Tol and P. Demeester, in a paper entitled xe2x80x9cAn improved technology for eliminating, polarization dispersion in integrated phasar demultiplexersxe2x80x9d, in Proc. 23.sup.rd Conf. on Optical Comm. (FCOC""97), pp. 3.83-3.86, Edinburgh, UK, 1997. In addition to increases complexity in fabrication process, the reduced cladding layer thickness in the polarization compensating rib/ridge waveguides resulted in a reduced lateral index contrast, and consequently increased phase errors due to enhanced coupling between adjacent waveguides. In order to avoid radiation loss due to reduced index contrast, the polarization compensating waveguides need to he implemented in straight waveguide section, which leads to an additional straight section length of the arrayed waveguides and consequently a larger device size:. Yet another alternative in the attempt to overcome polarization sensitivity is dispersion matching with adjacent diffraction orders which has been described by M. Zirngibl, C. H. Joyner, I. W. Stulz, Th. Gaigge and C. Dragone, in a paper entitled xe2x80x9cPolarization independent 8.times.8 waveguide grating multiplexer on InPxe2x80x9d, Electron. Lett., vol. 29, pp. 201-201, 1993, and by L. H. Spiekman, M. R. Amersfoort, A. H. de Vreede, F. P. G. M. van Ham, A. Kuntze, J. W. Pedersen, P. Demeester, and M. K. Smit, in a paper entitled; xe2x80x9cDesign and realization of polarization independent phased array wavelength demultiplexers using different array order for TE and TMxe2x80x9d, J. Lightwave Technol., vol. 14, pp. 991-995, 1996. Another approach is that of using layer structures with low birefringence by using thick guiding layer and low refractive index contrasts has been described by H. Bissessur, F. Gaborit, B. Martin, P. Pagnod-Rossiaux, J. L. Peyre and M. Renaud, in a paper entitled xe2x80x9c16 channel phased array wavelength demultiplexer on InP with low polarization sensitivityxe2x80x9d, Electron. Lett., vol, 30, pp. 336-337, 1994.
For diffraction grating based wavelength multi/demultiplexers, only the last two approaches are possible. In the polarization compensation method which attempts to match the TE and TM passband to two adjacent diffraction orders, the tree spectral range (FSR) of the grating needs to be chosen equal to the wavelength split between the to modes. In this case, the passband corresponding to the mth-order for TE will overlap with the (m-1)th order for TM. A severe drawback of this method is that the available FSR for WDA channels is limited by the polarization split, which is determined by the waveguide layer structure. It is usually limited to a few nanometers. A large polarization split is preferable in this case. In addition, since the polarization dispersion is very sensitive to the exact layer composition and thickness, it is difficult to obtain a good match due to the non-uniformity and non-reproducibility of wafer growths. Another method for achieving polarization insensitive operation in diffraction grating based wavelength multi/demultiplexer is to use a birefringence-reduced layer structure, combined with an input/output waveguide design for a flattened channel response. Polarization dispersion as small as 0.3-0.4 nm has been obtained with InGaAsP/InP double heterostructures as is described by J.-J. He. B. Lamontagne, A. Delage, I. Erickson, M. Davies, and E. S. Koteles, in a paper entitled xe2x80x9cMonolithic integrated wavelength demultiplexer based on a waveguide Rowland circle grating in lnGaAsP/InPxe2x80x9d, J. Lightwave Tech, vol. 16, pp. 631-638, 1998. Lower birefringence waveguides can be designed by using a thick guiding layer and low refractive index contrast between the guiding layer and the cladding layer. However, low index contrast InGaAsP/lnP layers are very difficult to grow in practice. One way to obtain low index contrast waveguides is to use homogenous InP with different doping levels for the guiding and cladding layers, as suggested by Gini, W. Hunziker, and H. Melchior, in a paper entitled xe2x80x9cPolarization independent WDM multiplexer/demultiplexer modulexe2x80x9d, J. Lightwave Tech, vol. 16, pp. 625-630, 1998, incorporated herein by reference. A somewhat polarization independent waveguide structure having a thick InP waveguide and etched according to a process disclosed therein is proposed. The authors disclose that the waveguide is approximately polarization independent. In reality, the elliptical mode of the waveguide disclosed as well as other factors result in substantial polarization dependence. The residual polarization dispersion is still as large as 0.1 nm. In U.S. Pat. No. 5,937,113 in the names of He et. al. And issued Aug. 10, 1999, a small perturbation in a waveguide structure formed by etching, doping, or ion implantation allows for a more polarization independent operation of the waveguide. Such a system is advantageous as polarity of signals propagating through the waveguide structure are insignificant to waveguide operation. However, the waveguide core layer is generally thin in order to keep the waveguide single-mode as required for the device application. This results in a large fiber-to-waveguide coupling loss due to the mode mismatching.
It would be advantageous to produce a symmetric, and large-size waveguide structure that is substantially polarization independent and at the same time has a good mode-matching with standard optical fiber.
It is an object of the invention to provide a semiconductor waveguide structure that is approximately symmetrical for enhancing polarization independence thereof and for improving the fiber-waveguide coupling efficiency.
In an attempt to overcome these and other limitations of the prior art, a homogeneous semiconductor waveguide structure is provided having a substrate, a core waveguide layer formed on the substrate and being a thick waveguide, and a further layer of substrate material on the core waveguide layer such that the resultant waveguide is substantially symmetrical to an optical signal propagating therein. According to an embodiment, the core waveguide material is undoped InP and the substrate is a doped InP substrate. Advantageously, an optical signal propagating in the core of the waveguide structure encounters a similar environment on each side of the core.
In an embodiment the invention provides a semiconductor wave guide structure comprising:
a substrate including a first doped cladding layer having a first index of refraction;
a thick undoped waveguide core layer disposed on the substrate in contact with the first doped cladding layer and in contact therewith, having a thickness of at least 2 xcexcm, and having an index of refraction higher than that of the first doped cladding layer; and,
a second doped cladding layer having an index of refraction similar to that of the first doped cladding layer and disposed on an opposing side of the waveguide cure layer to the first doped cladding layer.